Motion detectors often use microwave transceivers, passive infrared detectors, or both to sense motion. A microwave transceiver creates an energy field of microwave energy having a particular carrier frequency. The energy field is called the "field of view" of the transceiver. The microwave transceiver monitors this field of view and detects motion by phase changes in the received carrier frequency. When the phase of the received carrier frequency of the monitored signal changes, motion, such as the presence of the intruder, is detected.
One such motion detector is illustrated in FIG. 1. A motion detector 20 is mounted in the corner of a room. The motion detector includes at least a microwave transceiver 22 which creates a field of microwave energy 24. A fluorescent light fixture 26 is present in this field 24. The fixture 26 contains a fluorescent light tube 28. This tube includes gas which, when electrons are passed through it, generates light.
One problem which has been known to those skilled in the art is that fluorescent lights located within the microwave transceiver's field of view often cause false alarms. That is, the microwave transceiver "detects" motion even though there is no motion in the field. This is because the discharge of electrons in the fluorescent light tube (i.e., the motion of electrons through the gas in the light tube) gives a false motion signal by shifting the phase of the received carrier frequency of the monitored signal. This phase change in the received carrier frequency is caused by energy at multiples of the AC power line frequency. Most of this energy occurs at twice the AC power line frequency (i.e., the first harmonic). This energy causes the microwave transceiver 22 to falsely "detect" motion.
One known prior art solution to this problem is illustrated in FIG. 2. FIG. 2 is a block diagram of a motion detector 20. A microwave transceiver (DRO) 22 outputs to a doppler amplifier section 34 a signal at a doppler frequency. The doppler frequency represents the phase change in the received carrier frequency due to motion of an object in the field of view. The doppler amplifier section 34 amplifies the output of the DRO 32 for further processing. The amplified signal is then filtered through a hardware-based notch filter 36. The notch filter 36 is designed to have a center frequency at twice the AC power line frequency, i.e., the first harmonic of the main frequency. In the United States, where the electricity supplied by the AC power lines has a frequency of 60 Hz, the first harmonic of this frequency is at 120 Hz.
The signal output from the notch filter 36 is then applied to a processor 38, such as a microprocessor, to process the signal as required by the motion detector 20. The processor may, for example, determine that motion has been detected and, in response, generate a signal which may turn on a light or activate an alarm.
There are several drawbacks to this design. First, the notch filter 36 comprises discrete hardware components which are often imprecise and drift with temperature, time, and age. This imprecision causes the filter to have its "center frequency" at a frequency above or below 120 Hz, thus not effectively filtering out the first harmonic of the AC power line. Thus it fails to eliminate "false positive" signals. Second, the notch filter 36 only eliminates one component of the "false positive" signal. As mentioned above, the greatest energy emitted by the fluorescent light is at the first harmonic of the AC power lines, e.g., 120 Hz in the United States. However, the fluorescent light emits energy at the main AC power line frequency and other harmonics, as well. Thus, energy may also be emitted at 60 Hz, 180 Hz, 240 Hz, 300 Hz, etc. The prior art notch filter does not eliminate signals caused by the energy of the main frequency and these other harmonics. Third, to use the device in foreign countries having an AC power frequency other than 60 Hz, the hardware-based notch filter must be redesigned to have a different center frequency. For example, in the United Kingdom and other European countries, the AC power frequency line is 50 Hz. Thus, the filter must be designed to have a center frequency at 100 Hz to filter out the first harmonic of the 50 Hz AC power line frequency.
Thus, it is an object of the present invention to provide a filter for eliminating the effects of fluorescent lights on microwave transceivers.
It is another object of the present invention to provide a filter which will be precisely tuned to the frequencies to be eliminated.
It is a further object of the present invention to provide a filter which eliminates all signals transmitted due to the AC power lines including the center frequency and all its harmonics.
It is yet a further object of the present invention to provide a filter which is easily adaptable for use in countries having an AC power frequency different from the United States'.